Growth of nickel nanoparticles on NiO/Ni(0 0 1): Evidence of adsorbed oxygen on metal particles by metastable induced electron spectroscopy (MIES)

We present results – obtained with X-ray as well as ultraviolet photoelectron spectroscopy (XPS and UPS) and metastable induced electron spectroscopy (MIES) – for the deposition of nickel on a NiO film of about 10 monolayers. It was found that up to a coverage of 0.5 monolayers single atoms or non-metallic clusters with up to 7 nickel atoms are formed. A MIES reference spectrum for this situation has been reconstructed from the experimental spectral series and the surface density of states has been obtained by deconvolution of this spectrum. Above 0.5 monolayer nominal nickel coverage a metallic nickel overlayer is formed as indicated by UPS. However, only MIES with its perfect surface sensitivity provides direct experimental evidence that on top of the metallic nickel a small amount of oxygen is adsorbed which originates from the underlying oxide.